[jase (Jason)]

Originally Posted by Gama

Due to the certain views most people have about optical design, i though a post soley for optical discussions be started.

RC's are NOT the best optical design, contrary to many peoples beliefs. But by placing a lens correcting setup, it puts it close to perfect!. But then the scope becomes a Cataioptric (Cat).
The Corrected Dall Kirkham (CDK) is corrected to remove many abberations.
To an RC, the CDK is a better optical design..

Well, I can’t help starting without a diplomatic statement - the best optical design is one that meets your needs. I’ve done my fair share of studying optical statistics, ray-trace diagrams and the like. You begin to realise as you sift through the data that a perfect optical design does not exist. Sure, some are better than others for specific applications. There are differences between taking “pretty pictures” compared to scientific data acquisition.

Firstly, I would like to clarify I don’t have anything against corrector plate/lens designs. We should distinguish there is a difference between the two. A corrector plate resides in front of the primary mirror. A corrector lens is located after secondary mirror usually close to the focal plane. The latter is actually no different to an advanced focal reducer/field flattener such as those used in the Vixen VC200L and the Astro Systeme Austria (ASA) series.

Originally Posted by Gama

Telescope Type: ................Spherical: .................Coma: ..................Field Curvature

Originally Posted by Gama

Classical Cassegrain.................... Corrected ......................Has coma ........................Has Curvature

Corrected Dall-Kirkham ..............Corrected ......................Corrected ........................Corrected
Ritchy-Chretien ..........................Corrected ......................Corrected ........................Has Curvature
Schmidt-Cassegrain ....................Corrected ......................Has Coma .......................Has Curvature

Your table appears to be directly from Vixen’s info- http://vixenamerica.com/Pdf/Scopes/V..._SX_VC200L.pdf
Of course they are going to say it corrects everything, they’re trying to sell their product. It is disappointing when manufacturers make such optical design comparisons unless it’s an apples to apples case - such as a RC with Star-Instruments mirror sets vs. a RC with Astrosib mirror sets. In some cases, they’re just stating the stating the obvious.

Originally Posted by Gama

Further a mention was made on another post about spot diagrams
" More importantly, again we see a manufacturer (PlaneWave Instruments) who compares their products to the RC optical design through spot diagrams, etc. just like Meade and Anssen Technologies have done in the past. It is the statements and figures conveyed that have the potential to fool the prospective buyer"
This is partly correct, this is because manufacturers tend to only toot the good points and leave out the drawbacks. Problem is ALL manufacturers do it. Yes even RC Optical and RCOS, so we cant just say Meade or Anssen have done it in the past. Just to nip it in the bud, Anssen has a manufacturing problem, and nothing to do with the spot diagram or design of the scope. The scope is designed correctly, just other forces producing bad images. Further, the Spot diagram is the process of comparing telescope performance, and every telescope manufacturer designs their optical systems using Spot diagrams as its proof of performance.

Even the Vixen VC200L is a CDK type, so read about it.

That simply doesn't cut it. You buy a telescope of its optical quality/performance. It doesn't matter how good the design. If the manufacture can't deliver the indicated performance due to manufacturing defects the telescope is next to useless. Spot diagrams are only part of the optical testing equation.

What has happened to the Strehl ratio – the definite measurement in determining optical quality? Manufacturers are extremely shy in providing these details. Why is this so? Poor internal quality control, difficult to obtain consistency, all too hard and time consuming? Seriously when you’re entering into the high-end market, I’d be extremely sceptical in purchasing an OTA that didn’t provide this. It is not possible to purchase a new RC without receiving such information on your specific mirror set in the OTA you’ve purchased.

Originally Posted by Gama

Lastly, one question was asked about why do they prefer the RC over other designs.
Well, the RC only uses 2 mirrors, so you get NO loss or abberations introduced by passing light thru glass, plus ALL optical wavelenghts are also reflected.

I think you nailed this one nicely – be wary of corrector/lens designs in particular the coatings they use. Try imaging at UV wavelengths for photometry with some SCT’s for example, it’s near impossible because of the glass/coating combination. Thus it comes back to application – “pretty pictures” or scientific data acquisition.

If we purely go on what is happening in the market with high-end optical purchases, I’d like to challenge why there are more RC’s are out there than CDK’s. If the CDK design is indeed superior, why aren’t there more people using them? The cost is certainly CDK favourable considering a 20” CDK - US$32,500 and the 20” RCOS is US$54,300. There has clearly got to be a reason why many advanced amateurs, government agencies, etc go the RC path. Admittedly, this is a basic viewpoint, but if there isn’t a market then these companies would surely fold.